This invention relates to a method of applying phosphors and more particularly to methods and techniques for applying phosphors to provide intagliated phosphor screens.
Fiber optic phosphor screens are employed in a number of devices such as image intensifier tubes used in night vision systems. Certain of these screens are referred to as intagliated phosphor screens.
The basic function of a device as an image intensifier tube is the extension of the region of vision toward lower light levels in the domain of the electromagnetic spectrum. Such tubes extend visibility by gathering more light from the scene with an objective lens and by the use of a photocathode, which has a high quantum efficiency and broader spectral response, one gains increased acuity over viewing with the eye. Limitations of visibility occurring with the use of such tubes are the statistical variation of the photo-cathode current, the contrast sensitivity of the eye in the useful spatial frequency domain and the contrast transfer function of the image intensifier.
It is known that the field resolution is determined along with the other parameters by the contrast transfer function of the tube. More specifically, the higher the contrast function, the better is the probability of target recognition. The probability of target recognition is also proportional to the signal to noise ratio of the pattern under observation. A term designated as MTF (modulation transfer function) is a more frequently used image transfer function. The term has a special significance for an unmabiguous assessment of image quality. With the MTF, the resolution and contrast and thus the amount of perceivable detail and the sharpness of the image is perceived in the entire spatial frequency domain. The MTF of image tubes is a product of the sine wave response of the optical components of the tube, which components include the phosphor screen.
The MTF loss of conventional image screen tubes can be reduced by the use of intagliated fiber optic phosphor screens. It is known that when a phosphor screen is packed in the intagliated fiber optic plate, the MTF of the screened fiber optic plate approaches the MTF of the fiber optic substrate.
In such screens, the phosphor material is deposited in metalized etch pits of the fiber optic core glass and the entire screen surface is aluminized.
In any event, the fabrication of intagliated fiber optic screens according to prior art techniques experienced much difficulty.
Image phosphor screens are less than 10 microns thick and consist of relatively smooth layers of luminescent material. These phosphor screens are frequently water settled with phosphor particles that are one to five microns in size. The screens are coated with a thin, opaque film of aluminum (about one thousand angstroms thick).
In water settling, the phosphor particles are allowed to settle under the influence of gravity from a water suspension of phosphor powder to which potassium silicate or sodium silicate compounds are added as screen binders and sodium bicarbonate or barium acetate as an electrolyte. The electrolyte is a gelling agent which promotes adhesion between the individual phosphor granules and the substrate. It may be added to the settling water after the phosphor particles are settled. At the end of the settling period, the liquid is drained and the phosphor layer is air dried and baked. The screen is then placed in water and coated with a thin organic film by a water floatation technique. A drop of organic film solution is allowed to form a thin film on the top of the water which, by draining the water, settles on the screen. The screen is then aluminized and air baked to remove the organic film.
The aluminum film has a triple role; first, it holds the phosphor screen at a uniform potential by providing it with a conductive layer, second, it prevents light feedback from the phosphor screen to the photo-cathode, third, it serves as a reflector for the phosphor screen, thus improving its efficiency.
In certain cases, the screen is further dark aluminized in order to improve contrast by elimination of light reflections from the screen to the photocathode. A dark aluminum film is usually deposited by evaporation of aluminum in air at a pressure of about 10.sup.-1 torr.
Essentially, a basic problem of phosphor screen deposition onto the "honey combed" structure of the intagliated fiber optic structure is to uniformly compact the phosphor particles into the extremely small etch pits (9.9 micrometer holes). Conventional techniques as gravitational sedimentation of phosphor particles through a colloidal suspension onto the substrate surface is not efficient because the liquid improperly wets the substrate causing air bubbles and thus prevents the phosphor particles from entering many of the etch pits. Likewise, gravitational sedimentation produces a "fluffy" phosphor layer of low resolution.
It is therefore an object of the present invention to provide a process whereby phosphor particles can be accurately and reliably deposited on an intagliated substrate to thereby provide improved intagliated structures for use in image intensifiers, tubes and other applications as well.